Transmission liquid crystal display and method of forming the same

ABSTRACT

A method for cleaning a hole in a layered structure having a planarized transparent organic surface comprises the step of exposing said hole to sputtered particles or plasma particles in the presence of a transparent protection layer which covers said planarized transparent organic surface, except within said hole, for protecting said planarized transparent organic surface from said particles.

This application is a division of co-pending application Ser. No.10/675,958, filed on Oct. 2, 2003. Application Ser. No. 10/675,958 isthe divisional of co-pending application Ser. No. 09/836,506 filed onApr. 18, 2001, which claims priority of Japanese Application No.2000-117585 filed Apr. 19, 2000. The entire contents of each of theabove-identified applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transmission liquid crystal displayand a method of forming the same, and more particularly to atransmission liquid crystal display with an improved contact holestructure and a method of forming such contact holes therein.

2. Description of the Related Art

In recent years, liquid crystal displays have widely been used invarious fields due to their advantageous light weight and low powerconsumption. A transmission active matrix liquid crystal display is ahighly attractive liquid crystal display. Such a display has an activematrix substrate and an opposite substrate, which together define aninter-space filled with liquid crystal. The active matrix substrate hasplural thin film transistors, which serve as switching devices forswitching display pixels, wherein the display pixels are provided overthe active matrix substrate. The opposite substrate is transparent, forallowing transmission of light. It is important for such a display tohave improved contrast and color and also increased display screen area.

In order to achieve such improvements, it is necessary to increasetransmittivity of a back-light emitted from a back-illuminator, whichilluminates a liquid crystal panel.

Advanced liquid crystal displays reduced weight, thickness and powerconsumption. In order to reduce power comsumption, it is quite effectiveto save or reduce a power comsumption by the back-illuminator.

Regarding improved transmittivity of the back-light, it is effective toincrease an aperture efficiency of a display portion, which includespixels. In view of increasing the aperture efficiency, the display hasthe following structural elements.

The liquid crystal device has a planarized transparent organicinsulating film, which covers entire regions including theinterconnections coupled with the electrodes and the thin filmtransistors, wherein an aperture comprises a transparent film whichpermits light-transmission. An inter-layer insulator as a protector isfurther provided, which overlies the substrate and underlies theplanarized transparent organic insulating film. Transparent pixelelectrodes are provided over such a planarized transparent film. Contactholes are formed, which penetrate laminations of the planarizedtransparent organic insulating film and the inter-layer insulator, sothat contact plugs are then formed in the contact holes, whereby thetransparent pixel electrodes over the planarized transparent film areelectrically connected through the contact plugs to interconnections,which underlie the inter-layer insulator, wherein the interconnectionsare coupled with the electrodes of the thin film transistors over thesubstrate.

The above inter-layer insulator comprises an inorganic insulating film,whilst the planarized transparent film comprises an organic insulatingfilm. This means that the contact holes penetrate the laminations of theorganic and inorganic films. The contact holes are formed by an etchingprocess for selectively etching such laminations, before the contactholes are filled with the contact plugs by a sputtering process. Theetching process and the subsequent sputtering process, however, damagethe planarized transparent organic insulating film, whereby the film isdeteriorated in light-transmittivity. This means that thelight-transmittivity of the apertures is reduced.

Japanese laid-open patent publication No. 10-20342 discloses contactholes which penetrate laminations of an overlying photo-sensitiveorganic insulating layer and an underlying inorganic insulating layer. Aphoto-lithography technique may be utilized to form the contact holes inthe overlying photo-sensitive organic insulating layer, before a dryetching process is carried out with use of the processed overlying layeras a mask for forming the contact holes. The dry etching process uses anetching gas which contains carbon, fluorine and hydrogen in order toprevent deterioration of the overlying photo-sensitive organicinsulating layer and also prevent the underlying layer from side-etch.

Japanese laid-open patent publication No. 11-283934 discloses anotherconventional technique to prevent any substantal increase in aresistance value of the contact plug in the contact hole. Contact holesare formed in a transparent resin inter-layer insulator by an etchingprocess, by which residues remain on bottoms of the contact holes. Afurther sputter cleaning process is carried out to remove residues fromthe bottoms of the contact holes because the residues may increase theresistance, wherein the residues are removed by impacts of sputteredparticles in hydrogen, helium or nitrogen atmosphere.

FIG. 1A is a fragmentary cross sectional elevation view of such asputter cleaning process for removing the residues from the contactholes. The planarized top surface of the transparent organic film 8 isdirectly exposed to the sputtered particles, whereby the planarizedsurface becomes a rough surface 17 a, and an upper region 17 b of thetransparent organic film 8 is damaged and deteriorated in film quality.FIG. 1B is a fragmentary cross sectional elevation view of thetransparent organic film with the damaged upper region by the sputtercleaning process of FIG. 1A. The above organic film 8 has a C—C bondingstructure. In the upper region 17 b of the organic film 8, this C—Cbonding structure may be broken by the sputtered particles, whereby themolecular structure of the upper region 17 b is changed, for example,side chains of the molecule structure are broken. As a result, the aboverough surface 17 a is formed. This rough surface 17 a causes aremarkable decrease in light-transmittivity of the transparent organicfilm 8.

In the above circumstances, it would be advantageous to develop a noveltransmission liquid crystal display and method of forming the same freefrom the above problems.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a noveltransmission liquid crystal display and method of forming the same freefrom the above problems.

In accordance with the present invention, a method for cleaning a holein a layered structure having a planarized transparent organic surfacecomprises the step of exposing said hole to sputtered particles orplasma particles in the presence of a transparent protection layer whichcovers said planarized transparent organic surface, except within saidhole, for protecting said planarized transparent organic surface fromsaid particles.

The above and other objects, features and advantages of the presentinvention will be apparent from the following descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments according to the present invention will bedescribed in detail with reference to the accompanying drawings.

FIG. 1A is a fragmentary cross sectional elevation view of such asputter cleaning process for removing the residues from the contactholes.

FIG. 1B is a fragmentary cross sectional elevation view of thetransparent organic film with the damaged upper region by the sputtercleaning process of FIG. 1A.

FIG. 2 is a fragmentary cross sectional view of a transparent liquidcrystal display in accordance with the present invention.

FIGS. 3A through 3K are fragmentary cross sectional views of liquidcrystal displays in sequential steps involved in the novel manufacturingmethod in a first preferred embodiment in accordance with the presentinvention.

FIG. 4 is a fragmentary cross sectional view of alternatively availableplasma ashing process in a modified embodiment in accordance with thepresent invention.

FIGS. 5A through 5E are fragmentary cross sectional views of liquidcrystal displays in different sequential steps in the alternativeprocesses from the sequential steps of FIGS. 3A through 3K.

FIG. 6 is a fragmentary cross sectional view of alternatively availableplasma ashing process in a modified embodiment in accordance with thepresent invention.

DETAILED DESCRIPTION

A first aspect of the present invention is a method for cleaning a holein a layered structure having a planarized transparent organic surface,comprising the step of exposing said hole to sputtered particles orplasma particles in the presence of a transparent protection layer whichcovers said planarized transparent organic surface, except within saidhole, for protecting said planarized transparent organic surface fromsaid particles.

The transparent protection layer protects the planarized transparentorganic surface from the sputtered particles in the sputter cleaningprocess or the plasma particles in the plasma ashing process forremoving residues from the hole, whereby the planarized transparentorganic surface is free from any damage and any deterioration in filmquality by breaking molecular structure. After such a cleaning process,the layered structure keeps the planarized transparent organic surfacewhich is free of any substantive or large roughness. This planarizedtransparent organic surface provides a high light-transmittivity.

It is preferable that the transparent protection layer has alight-transmittivity of not less than 90% under a condition of avertical incident of a light having a wavelength in the range of 400-800nanometers, provided that the light-transmittivity is represented by aratio in quantity of a transmitted light to a vertically incident light.

It is also preferable that the transparent protection layer has agas-permeability. After the transparent protection layer has beenformed, a heat treatment may be carried out, whereby a gas is generatedfrom the transparent organic material under the transparent protectionlayer. Since, however, the transparent protection layer has thegas-permeability, the generated gas is permeated through the transparentprotection layer and discharged to an atmosphere, thereby causing noaccumulation of the generated gas on the interface between thetransparent protection layer and the planarized transparent organicsurface. Such no accumulation of the generated gas does not cause thetransparent protection layer to be peeled from the planarizedtransparent organic surface. It is preferable for achieving the aboveeffects that the transparent protection layer has a relative filmdensity in the range of 50-90%.

It is preferable that the transparent protection layer has a thicknessof at least about 15 nanometers for exhibiting a sufficient effect ofprotecting the planarized transparent organic surface from the sputteredparticles or the plasma particles. The thickness is more preferably inthe range of at least 20 nanometers to at most about 40 nanometers. Ifthe thickness of the transparent protection layer is much thinner than15 nanometers, then it is possible that the transparent protection layerdoes not exhibit the sufficient effect of protecting the planarizedtransparent organic surface from the sputtered particles or the plasmaparticles. If the thickness of the transparent protection layer isfurther increased from 40 nanometers, then no further advantage can beobtained.

It is also preferable that the layered structure comprises laminationsof an underlying inorganic inter-layer insulator and an overlyingtransparent organic insulating layer having the planarized transparentorganic surface, provided that the hole penetrates the overlyingtransparent organic insulating layer and the underlying inorganicinter-layer insulator. The underlying inorganic inter-layer insulatorhas a surface which varies in level such as a stepped surface, whilstthe overlying transparent organic insulating layer has the planarizedtransparent organic surface.

The hole may comprise either a contact hole or a through hole.

The above cleaning process may comprise either a sputter cleaningprocess with the sputtered particles or the plasma-ashing process withthe plasma particles.

A second aspect of the present invention is a method of forming a holein a layered structure having a planarized transparent organic surface.The method comprises the steps of: forming a transparent protectionlayer which covers the planarized transparent organic surface; carryingout an anisotropic etching for selectively etching the transparentprotection layer and the layered structure to form a hole in the layeredstructure; and exposing said hole to sputtered particles or plasmaparticles in the presence of a transparent protection layer which coverssaid planarized transparent organic surface, except within said hole,for protecting said planarized transparent organic surface from saidparticles.

This second aspect of the present invention has the same characteristicsdescribed above in connection with the cleaning method of the firstaspect of the present invention.

A third aspect of the present invention is a method of forming a hole ina layered structure having a planarized transparent organic surface. Themethod comprises the steps of: carrying out an anisotropic etching forselectively etching the layered structure to form a hole in the layeredstructure; forming a transparent protection layer on the planarizedtransparent organic surface and on a bottom and side walls of the hole;selectively removing the transparent protection layer from the bottomand the side walls of the hole, so as to leave the transparentprotection layer on the planarized transparent organic surface only; andexposing said hole to sputtered particles or plasma particles in thepresence of a transparent protection layer which covers said planarizedtransparent organic surface, except within said hole, for protectingsaid planarized transparent organic surface from said particles.

This third aspect of the present invention has the same characteristicsdescribed above in connection with the cleaning method of the firstaspect of the present invention.

A fourth aspect of the present invention is a layered structurecomprising: a transparent organic layer having a planarized transparentorganic surface and a hole; and a first transparent layer overlying theplanarized transparent organic surface only except within the hole.

It is preferable to further comprise a second transparent layer, whichhas an electrical conductivity and extends over the transparentprotection layer and on a bottom and side walls of the hole. The firstand second transparent layers may be made of the same martial. Such amartial may be either indium tin oxide or silicon dioxide. The firsttransparent layer may comprise a transparent protection layer, and thesecond transparent layer may comprise a transparent pixel electrodelayer.

It is preferable to further comprise an orientation film extending overthe transparent pixel electrode layer and within the hole, wherein theorientation film is in contact with a liquid crystal.

The effect of providing the transparent protection layer which coversthe planarized transparent organic surface except within the hole is thesame as described in connection with the above first aspect of thepresent invention.

A fifth aspect of the present invention is a transparent liquid crystaldisplay comprising: a first substrate; an inorganic inter-layerinsulator layer extending adjacent to the first substrate; a transparentorganic layer having a planarized transparent organic surface and ahole, the transparent organic layer extending adjacent to the firstsubstrate; a transparent protection layer covering the planarizedtransparent organic surface except within the hole; a transparent pixelelectrode layer extending adjacent to the transparent protection layerand on a bottom and side walls of the hole; a first orientation filmextending over the transparent pixel electrode layer and within thehole; a liquid crystal adjacent to the first orientation film; a secondorientation film adjacent to the liquid crystal; an opposite electrodefilm adjacent to the second orientation film; and a second substrateadjacent to the opposite electrode film.

This fifth aspect of the present invention has the same characteristicsdescribed above in connection with the first and fourth aspects of thepresent invention.

Throughout this specification, the word “sputter cleaning process” meansa sputter etching process for removing the residues from the hole withsputtered particles.

PREFERRED EMBODIMENTS First Embodiment

A first embodiment according to the present invention will be describedin detail with reference to the drawings. FIG. 2 is a fragmentary crosssectional view of a transparent liquid crystal display in accordancewith the present invention. The transparent liquid crystal display hasfirst and second insulating substrates 16 a and 16 b. A thin filmtransistor silicon layer 1 is selectively provided on a predeterminedregion of a top surface of the first insulating substrate 16 a. Thesilicon layer 1 for thin film transistor has a thickness in the range of30-100 nanometers. The silicon layer 1 may comprise a single crystalsilicon layer or a polycrystal silicon layer.

A gate insulating film 2 is provided, which extends over the thin filmtransistor silicon layer 1 and the remaining region of the top surfaceof the first insulating substrate 16 a. The gate insulating film 2 has athickness in the range of 10-100 nanometers. The gate insulating film 2may be made of a silicon-based insulator such as a silicon dioxide. Agate electrode interconnection 3 is selectively provided on the gateinsulating film 2, so that the gate electrode interconnection 3 ispositioned indirectly over the silicon layer 1, wherein the gateelectrode interconnection 3 is separated from the silicon layer 1 by thegate insulating film 2. The gate electrode interconnection 3 may have asingle layered structure, or a laminated structure. The gate electrodeinterconnection 3 may comprise any of various metal layers, for example,Al, Mo, W, and Ta layers and various metal silicide layers, for example,AlSi, AlCuSi, TiSi, MoSi and WSi layers alone or in combination.

A first inter-layer insulator 4 is provided, which extends over a topsurface of the gate insulating film 2 and over the gate electrodeinterconnection 3. The first inter-layer insulator 4 may be made of asilicon based insulator such as silicon dioxide. The first inter-layerinsulator 4 may have a thickness in the range of 100-500 nanometers. Apair of contact holes 5 a and 5 b is formed in the lamination of thefirst inter-layer insulator 4 and the gate insulating film 2, so thatthe contact holes 5 a and 5 b reach parts of the top surface of thesilicon layer 1.

Source and drain electrodes 6 a and 6 b are selectively provided on, atop surface of the first inter-layer insulator 4, wherein the source anddrain electrodes 6 a and 6 b are electrically connected through firstand second contact plugs in the contact holes 5 a and 5 b to the siliconlayer 1. The source and drain electrodes 6 a and 6 b are connected tofirst level interconnection layers 60 which extends over the top surfaceof the first inter-layer insulator 4. The source and drain electrodes 6a and 6 b, the first and second contact plugs and the first levelinterconnection layers 60 may comprise a unitary-formed conductive layersuch as an aluminum layer.

A second inter-layer insulator 7 is provided, which extends over the topsurface of the first inter-layer insulator 4 and also over the sourceand drain electrodes 6 a and 6 b and the first level interconnectionlayers 60. The second inter-layer insulator 7 may be made of aninorganic insulating material, for example, silicon-based insulator suchas silicon dioxide. The second inter-layer insulator 7 may have athickness in the range of 300-800 nanometers. The gate insulating layer2 has a single-stepped surface. The first inter-layer insulator 4 has adual-stepped surface. The second inter-layer insulator 7 has atriplet-stepped surface. As described above, the second inter-layerinsulator 7 may be made of the inorganic insulating material.

A transparent organic insulating layer 8 having a planarized top surfaceis provided over the second inter-layer insulator 7. It is possible thatthe transparent organic insulating layer 8 is provided directly over thetop surface of the first inter-layer insulator 4 and also over thesource and drain electrodes 6 a and 6 b and the first levelinterconnection layers 60 without providing the second inter-layerinsulator 7. It is, however, preferable that the second inter-layerinsulator 7 is provided directly over the top surface of the firstinter-layer insulator 4 and also over the source and drain electrodes 6a and 6 b and the first level interconnection layers 60, and thetransparent organic insulating layer 8 having a planarized top surfaceis provided over the second inter-layer insulator 7. A contact hole 10is formed in the lamination of the transparent organic insulating layer8 and the second inter-layer insulator 7, so that the contact hole 10reaches a part of a top surface of the first level interconnection layer60.

A transparent protection layer 9 a is provided on the planarized topsurface of the transparent organic insulating layer 8, provided that thetransparent protection layer 9 a does not extend on side walls and abottom of the contact hole 10. The transparent protection layer 9 a maybe made of indium tin oxide. The transparent protection layer 9 a mayhave a light-transmittivity of not less than 90% under a condition of avertical incident of a light having a wavelength in the range of 400-800nanometers, provided that the light-transmittivity is represented by aratio in quantity of a transmitted light to a vertically incident light.

The transparent protection layer 9 a may also have a gas-permeability.After the transparent protection layer has been formed, a heat treatmentmay be carried out, whereby a gas is generated from the transparentorganic material under the transparent protection layer. Since, however,the transparent protection layer has the gas-permeability, the generatedgas is permeated through the transparent protection layer and dischargedto an atmosphere, thereby causing no accumulation of the generated gason the interface between the transparent protection layer and theplanarized transparent organic surface. Such no accumulation of thegenerated gas does not cause the transparent protection layer to bepeeled from the planarized transparent organic surface. For achievingthe above effects, the transparent protection layer 9 a may have arelative film density in the range of 50-90%, wherein the relative filmdensity is represented by a volume ratio except for cavities and voidsin the film.

The transparent protection layer 9 a may have a thickness of at leastabout 15 nanometers for exhibiting a sufficient effect of protecting theplanarized top surface of the transparent organic insulating layer 8from sputtered particles in the sputter cleaning process or plasmaparticles in the plasma ashing process. The thickness is more preferablyin the range of at least about 20 nanometers to at most about 40nanometers. If the thickness of the transparent protection layer is muchthinner than 15 nanometers, then it is possible that the transparentprotection layer does not exhibit the sufficient effect of protectingthe planarized top surface of the transparent organic insulating layer 8from the sputtered particles or the plasma particles. If the thicknessof the transparent protection layer 9 a is further increased from 40nanometers, then no further advantage can be obtained.

A transparent pixel electrode layer 12 is further provided on a topsurface of the transparent protection layer 9 a and on the side wallsand the bottom of the contact hole 10, so that the transparent pixelelectrode layer 12 is in contact with the top surface of the first levelinterconnection layer 60, whereby the transparent pixel electrode layer12 is electrically connected through the first level interconnectionlayer 60 to the drain electrode 6 b. Preferably, the transparent pixelelectrode layer 12 may be made of the same material as the transparentprotection layer 9 a, for example, indium tin oxide. The transparentpixel electrode layer 12 may have a thickness in the range of 30-100nanometers.

A first orientation film 13-1 is provided on the top surface of thetransparent pixel electrode layer 12 and within the contact hole 10. Thedisplay has an opposite substrate 16 b. An opposite electrode 14 isprovided on a surface of a second insulating substrate 16 b. A secondorientation film 13-2 is provided on the opposite electrode 14. A liquidcrystal layer 15 is interposed between the first and second orientationfilms 13.

The structural characteristic of the display is the transparentprotection layer 9 a as described above. The transparent protectionlayer 9 a is provided for protecting the planarized transparent organicsurface from the sputtered particles in the sputter cleaning process forremoving residues from the contact hole 10, whereby the planarizedtransparent organic surface is free from any damage and anydeterioration in film quality by breaking molecular structure. Aftersuch a cleaning process, the layered structure keeps the planarizedtransparent organic surface which is free of any substantive or largeroughness. This planarized transparent organic surface provides a highlight-transmittivity.

The description will focus on the manufacturing process with referenceto FIGS. 3A through 3K, which are fragmentary cross sectional views ofliquid crystal displays in sequential steps involved in the novelmanufacturing method in this first embodiment.

With reference to FIG. 3A, a silicon layer having a thickness in therange of 30-100 nanometers is deposited over entire regions of the firstinsulating substrate 16 a by either a chemical vapor deposition methodor a sputtering method. A photo-resist mask is formed on a selectedregion of a top surface of the silicon layer for subsequent anisotropicetching to the silicon layer with the photo-resist mask, thereby to forma thin film transistor silicon layer 1, which overlies on apredetermined region of the top surface of the first insulatingsubstrate 16 a. The silicon layer 1 may comprise a single crystalsilicon layer or a polycrystal silicon layer. The used photo-resist maskis removed.

A gate insulating film 2 having a thickness in the range of 10-100nanometers is entirely deposited by a chemical vapor deposition methodor a sputtering method on the thin film transistor silicon layer 1 andthe top surface of the substrate 16 a. A metal or alloy film having athickness in the range of 50-300 nanometers is non-selectively depositedby a chemical vapor deposition method or a sputtering method on entireregions of the top surface of the gate insulating film 2. A photo-resistmask is formed on a selected region of a top surface of the metal oralloy film for subsequent anisotropic etching to the metal or alloy filmwith the photo-resist mask, thereby to form a gate electrode 3 on apredetermined region of the top surface of the gate insulating film 2.The used photo-resist film is removed.

With reference to FIG. 3B, a first inter-layer insulator 4 having athickness in the range of 100-500 nanometers is non-selectivelydeposited by a chemical vapor deposition method or a sputtering methodover the gate electrode 3 and the gate insulting film 2. The firstinter-layer insulator 4 has a stepped surface which comprisesthree-level stages. A photo-resist mask is formed on a selected regionof a top surface of the first inter-layer insulator 4 for subsequentanisotropic etching to the first inter-layer insulator 4 with thephoto-resist mask, thereby to form a pair of contact holes 5 a and 5 bin the lamination of the overlying first inter-layer insulator 4 and theunderlying gate insulating film 2, so that the contact holes 5 a and 5 breach parts of the top surface of the silicon layer 1. The usedphoto-resist film is removed.

With reference to FIG. 3D, a metal layer such as an aluminum layer isnon-selectively deposited by a chemical vapor deposition method or asputtering method over a top surface of the first inter-layer insulator4 and within the contact holes 5 a and 5 b. A photo-resist mask isformed on a selected region of the top surface of the metal layer forsubsequent anisotropic etching to the metal layer with the photo-resistmask, thereby to form source and drain electrodes 6 a and 6 b withcontact plugs and first level interconnections 60, wherein the sourceand drain electrodes 6 a and 6 b are electrically connected through thefirst and second contact plugs in the contact holes 5 a and 5 b to thesilicon layer 1. The first level interconnections 60 extend over the topsurface of the first inter-layer insulator 4.

With reference to FIG. 3E, a second inter-layer insulator 7 having athickness in the range of 300-800 nanometers is non-selectivelydeposited by a chemical vapor deposition method or a sputtering method,wherein the second inter-layer insulator 7 overlies the top surface ofthe first inter-layer insulator 4 and also over the source and drainelectrodes 6 a and 6 b and the first level interconnection layers 60.The second inter-layer insulator 7 may be made of an inorganicinsulating material, for example, silicon-based insulator such assilicon dioxide. The second inter-layer insulator 7 has atriplet-stepped surface which comprises four-level stages.

With reference to FIG. 3F, a transparent organic insulating layer 8having a planarized top surface is non-selectively formed by aspin-coating method over entire regions of the second inter-layerinsulator 7.

With reference to FIG. 3G, a transparent protection layer 9 a isnon-selectively formed by a sputtering method or a chemical vapordeposition method on entire regions of the planarized top surface of thetransparent organic insulating layer 8. The transparent protection layer9 a may be made of indium tin oxide. The transparent protection layer 9a may have a light-transmittivity of not less than 90% under a conditionof a vertical incident of a light having a wavelength in the range of400-800 nanometers, formed that the light-transmittivity is representedby a ratio in quantity of a transmitted light to a vertically incidentlight. The transparent protection layer 9 a may also have agas-permeability. After the transparent protection layer has beenformed, a heat treatment may be carried out, whereby a gas is generatedfrom the transparent organic material under the transparent protectionlayer. Since, however, the transparent protection layer has thegas-permeability, the generated gas is permeated through the transparentprotection layer and discharged to an atmosphere, thereby causing noaccumulation of the generated gas on the interface between thetransparent protection layer and the planarized transparent organicsurface. Such no accumulation of the generated gas does not cause thetransparent protection layer to be peeled from the planarizedtransparent organic surface. For achieving the above effects, thetransparent protection layer 9 a may have a relative film density in therange of 50-90%, wherein the relative film density is represented by avolume ratio except for cavities and voids in the film. If thetransparent protection layer 9 a is formed by the sputtering method, itis preferable that a sputter target has a similar relative density tothe transparent protection layer 9 a. If the transparent protectionlayer 9 a is formed by a plasma enhanced chemical vapor deposition, thenthe film density can be reduced by reducing temperature, for example, atmost 300° C., and also increasing pressure, for example, at least 1 Pa.The transparent protection layer 9 a may be made of either indium tinoxide or silicon dioxide.

The transparent protection layer 9 a may have a thickness of at leastabout 15 nanometers for exhibiting a sufficient effect of protecting theplanarized top surface of the transparent organic insulating layer 8from sputtered particles in the sputter cleaning process or plasmaparticles in the plasma ashing process. The thickness is more preferablyin the range of at least about 20 nanometers to at most about 40nanometers. If the thickness of the transparent protection layer is muchthinner than 15 nanometers, then it is possible that the transparentprotection layer does not exhibit the sufficient effect of protectingthe planarized top surface of the transparent organic insulating layer 8from the sputtered particles or the plasma particles. If the thicknessof the transparent protection layer 9 a is further increased from 40nanometers, then no further advantage can be obtained.

With reference to FIG. 3H, a photo-resist mask is formed on a selectedregion of the top surface of the transparent protection layer 9 a forsubsequent anisotropic etching, with the photo-resist mask, tolaminations of the transparent protection layer 9 a, the transparentorganic insulating layer 8 and the second inter-layer insulator 7,thereby to form a contact hole 10 a in such laminations, so that thecontact hole 10 a reaches a part of a top surface of the first levelinterconnection layer 60. During the anisotropic etching process,residues 11 a reside in the contact hole 10 a.

With reference to FIG. 31, a sputter cleaning process is carried out forremoving the residues 11 a from the contact hole 10 with sputteredparticles. This sputter cleaning process is equivalent in its technicalmeaning to the sputter etching process. During the sputter cleaningprocess, the transparent protection layer 9 a protects the planarizedsurface of the transparent organic insulating layer 8 from the sputteredparticles, so that the planarized surface is free from any damage andany deterioration in film quality by breaking molecular structure. Theplanarized surface of the transparent organic insulating layer 8 is freeof any substantive or large roughness. Such a planarized transparentorganic surface provides a high light-transmittivity.

FIG. 4 is a fragmentary cross sectional view of alternatively availableplasma ashing process. For the cleaning process, a plasma ashing processis also available instead of the sputter etching process. During theplasma ashing process, the transparent protection layer 9 a alsoprotects the planarized surface of the transparent organic insulatinglayer 8 from the plasma particles, so that the planarized surface isfree from such damage and deterioration in film quality.

With reference back to FIG. 3J, a transparent pixel electrode layer 12having a thickness in the range of 30-100 nanometers is non-selectivelydeposited by a chemical vapor deposition or a sputtering method, whereinthe transparent pixel electrode layer 12 overlies entire regions of atop surface of the transparent protection layer 9 a and further extendson the side walls and the bottom of the cleaned contact hole 10, so thatthe transparent pixel electrode layer 12 is in contact with the topsurface of the first level interconnection layer 60, whereby thetransparent pixel electrode layer 12 is electrically connected throughthe first level interconnection layer 60 to the drain electrode 6 b. Thetransparent pixel electrode layer 12 is made of indium tin oxide whichis transparent and electrically conductive.

With reference to FIG. 3K, the know fabrication processes are carriesout to complete the liquid crystal display.

As described above, the contact hole 10 is formed by the anisotropyetching to the above three laminated layers 9 a, 8 and 7, which arehowever, different in etching rate. Such differences cause undesirableside etching of the transparent organic insulating layer 8, whereby thetransparent protection layer 9 a overhangs. In this case, it ispreferable for preventing discontinuation of the transparent pixelelectrode layer 12 that the transparent protection layer 9 a is made ofthe same material as the transparent pixel electrode layer 12.

Not only indium tin oxide but also silicon dioxide are available for thetransparent protection layer which is air-permeable. If the transparentprotection layer is formed by the sputtering method, the use of indiumtin oxide is suitable for obtaining a lower film density as compared tosilicon dioxide.

The above gate electrode film may be formed at a low temperature of atmost 350° C. The transparent organic insulating layer 8 may be formed atmost 250° C. The transparent protection layer 9 a is formed at most 200°C. to keep high light-transmittivity.

If silicon dioxide is used for the transparent protection layer, ahigher etching stopper function in the sputter etching process can beobtained as compared to indium tin oxide, resulting in a reduced leveldifference which might suppress reverse-tilt of the liquid crystal,wherein plural liquid crystal domains co-exist, which have differenttilting directions, thereby making it difficult to obtain a uniform viewangle dependency of the display.

Further, in view of the high light-transmittivity, it is preferable thatthe transparent protection layer 9 a is made of the same material as thetransparent pixel electrode layer 12.

In order to prevent the transparent protection layer 9 a fromoverhanging, the above described sequential processes may partially bemodified.

FIGS. 5A through 5E are fragmentary cross sectional views of liquidcrystal displays in different sequential steps in the alternativeprocesses from the sequential steps of FIGS. 3A through 3K.

After the step of FIG. 3F, as shown in FIG. 5A, a photo-resist mask isformed on a selected region of the top surface of the transparentprotection layer 9 a for subsequent anisotropic etching, with thephoto-resist mask, to laminations of the transparent organic insulatinglayer 8 and the second inter-layer insulator 7, thereby to form acontact hole 10 b in such laminations, so that the contact hole 10 breaches a part of a top surface of the first level interconnection layer60. During the anisotropic etching process, residues 11 b reside in thecontact hole 10 b.

As shown in FIG. 5B, a transparent protection layer 9 a isnon-selectively formed by a sputtering method or a chemical vapordeposition method on entire regions of the planarized top surface of thetransparent organic insulating layer 8 and also on side walls and abottom of the contact hole 10 b.

As shown in FIG. 5C, the transparent protection layer 9 a is selectivelyetched from the contact hole 10 b, so that the transparent protectionlayer 9 a remains only over the planarized top surface of thetransparent organic insulating layer 8.

As shown in FIG. 5D, a sputter cleaning process is carried out forremoving the residues 11 b from the contact hole 10 with sputteredparticles. This sputter cleaning process is equivalent in its technicalmeaning to the sputter etching process. During the sputter cleaningprocess, the transparent protection layer 9 a protects the planarizedsurface of the transparent organic insulating layer 8 from the sputteredparticles, so that the planarized surface is free from any damage andany deterioration in film quality by breaking molecular structure. Theplanarized surface of the transparent organic insulating layer 8 is freeof any substantive or large roughness. Such a planarized transparentorganic surface provides a high light-transmittivity.

FIG. 6 is a fragmentary cross sectional view of alternatively availableplasma ashing process. For the cleaning process, a plasma ashing processis also available instead of the sputter etching process. During theplasma ashing process, the transparent protection layer 9 a alsoprotects the planarized surface of the transparent organic insulatinglayer 8 from the plasma particles, so that the planarized surface isfree from such damage and deterioration in film quality.

As shown in FIG. 5E, a transparent pixel electrode layer 12 having athickness in the range of 30-100 nanometers is non-selectively depositedby a chemical vapor deposition or a sputtering method, wherein thetransparent pixel electrode layer 12 overlies entire regions of a topsurface of the transparent protection layer 9 a and further extends onthe side walls and the bottom of the cleaned contact hole 10, so thatthe transparent pixel electrode layer 12 is in contact with the topsurface of the first level interconnection layer 60, whereby thetransparent pixel electrode layer 12 is electrically connected throughthe first level interconnection layer 60 to the drain electrode 6 b. Thetransparent pixel electrode layer 12 is made of indium tin oxide whichis transparent and electrically conductive.

In accordance with the modified sequential processes, the contact hole10 is formed before the transparent protection layer 9 a is formed, sothat the transparent protection layer is free of any overhanging.

Although the invention has been described above in connection withseveral preferred embodiments therefor, it will be appreciated thatthose embodiments have been provided solely for illustrating theinvention, and not in a limiting sense. Numerous modifications andsubstitutions of equivalent materials and techniques will be readilyapparent to those skilled in the art after reading the presentapplication, and all such modifications and substitutions are expresslyunderstood to fall within the true scope and spirit of the appendedclaims.

1. A display device comprising: pixel transistors; a transparent organicinsulating film covering said pixel transistors, and said transparentorganic insulating film having contact holes; and transparent pixelelectrodes extending over said transparent organic insulating film andwithin said contact holes and said transparent pixel electrodes being incontact with electrodes of said pixel transistors, wherein each of saidtransparent pixel electrodes has a variation in thickness, so that saidthickness of said electrode over said transparent organic insulatingfilm is thicker than said electrode within said contact hole.
 2. Themethod as claimed in claim 1, wherein said electrodes has adouble-layered structure over said transparent organic insulating film,and has a single-layered structure within said contact hole.